Degradation of the Urease Inhibitor NBPT as Affected by Soil pH

Abstract

Soil pH is an important property affecting N-(n-butyl) thiophosphoric triamide (NBPT) inhibition of urease. Only a few studies have followed NBPT decay kinetics by making direct measurements of this molecule, and its oxygen analog (NBPTo) in soil or buffer mediums. This study characterized NBPT metabolism under biotic and abiotic conditions including the appearance of NBPTo and decay products under a pH gradient. Chemical buffer incubations were conducted at pH 4.0, 5.0, 6.0, and 7.2, and degradation was quantified using ¹H-NMR. Soil incubations were conducted at pH 5.1, 6.1, 7.6, and 8.2 with sterilized (autoclaved) and non-sterilized soil collected from an agricultural field; and NBPT degradation and metabolism products were measured with high-preformance liquid chromatography (HPLC)-mass spectroscopy (MS). We found NBPT and NBPTo followed pseudo first-order decay kinetics in chemical buffers, and NBPT exhibited exponential decay patterns in soil with n-butylamine being a primary reaction product. The NBPT decay constants in buffers and soil were larger under acidic conditions and became progressively smaller as pH rose. Calculated NBPT half-life in non-sterilized soil was 0.07, 0.59, 2.70, and 3.43 d at pH 5.1, 6.1, 7.6, and 8.2, respectively. Soil sterilization increased these half-lives by 0.13, 0.55, 1.03, and 4.07 d, respectively. We found NBPTo to be more sensitive than NBPT to hydrolysis as result of the greater electrophilicity of the P atom. Chemical hydrolysis will likely be a large contributing factor to NBPT and NBPTo degradation in acidic to slightly alkaline soils (pH 5.1–7.6), while microbial degradation will likely become more important under more strongly alkaline conditions (pH 8.2).